1. Field of the Invention
The present invention relates to an improved process for inhibiting the formation and deposition of polymer-based fouling materials after caustic scrubbing of gaseous or liquid hydrocarbon streams contaminated with oxygenated materials with a basic washing solution having a pH&gt;7.
2. Background
Manufacture of ethylene and other olefins entails the use of pyrolysis or "cracking" furnaces to manufacture olefins from various gaseous and liquid petroleum feed stocks. Typical gaseous feed stocks include ethane, propane, butane and mixtures thereof. Typical liquid feed stocks include naphthas, kerosene, gas oil and crude oil.
In cracking operations, such as the pyrolytic cracking of ethane, propane, and naphtha to produce olefins, oxygenated compounds, including carbonyl compounds, are formed. The amount of carbonyl compounds, such as aldehydes and ketones, formed in such operations can vary widely, but is typically about 1-100 ppm in the hydrocarbon stream, with concentrations as high as 1000 ppm occasionally being encountered because of the utilization of various feed stocks and cracking temperatures.
Following cracking of the petroleum feed stock, the hydrocarbon effluent from the cracking furnace is subjected to a caustic, or basic, wash to remove various contaminants. Generally, this entails contacting in a caustic wash tower a basic washing solution (e.g., an aqueous solution having a pH&gt;7) with a gaseous or liquid hydrocarbon stream to remove acidic components, such as hydrogen sulfide and carbon dioxide, and oxygenated compounds, such as carbonyl compounds. The oxygenated compounds, however, undergo polymerization in the presence of the caustic scrubbing or basic wash conditions. Deposition of the polymer leads to fouling. Eventually, depending on the polymer deposition rate, the unit must be shut down for cleaning--obviously a costly operation. Consequently, methods of preventing fouling, or at least significantly reducing the rate of fouling are constantly being sought. Basic wash systems which require treatment to inhibit polymer-based fouling include amine acid gas scrubbers (e.g., MEA, DEA, isopropyl amine, butyl amine, etc.) and caustic wash systems.
In particular, during ethylene production, the major component of the oxygenated compounds contaminating the ethylene effluent from the cracking furnace is acetaldehyde. The ethylene effluent from the cracking furnace, containing acetaldehyde, is washed in a caustic tower. On contact with a caustic solution, acetaldehyde undergoes multiple base catalyzed Aldol condensation reactions. This results in formation of a water insoluble polymer which can coat the surfaces of the caustic tower and reduce operation efficiency.
In some ethylene production units, an amine acid gas scrubber is used in front of the caustic tower to remove most of the acid gases. On contact with an amine solution, as on contact with a caustic solution, acetaldehyde undergoes base catalyzed Aldol condensation reactions. The result of these Aldol reactions is formation of a water insoluble polymer.
At some ethylene manufacturing facilities, a vinyl acetate plant is also present. Ethylene is used in the vinyl acetate production process. Unreacted ethylene is recovered by distillation and sent back through the ethylene unit fractionation train. Vinyl acetate can be entrained with the unreacted ethylene and enter the fractionation train. When vinyl acetate reaches the caustic tower it is hydrolyzed to produce a salt of acetic acid and vinyl alcohol. Vinyl alcohol tautomerizes to acetaldehyde, a source of the fouling polymer.
Current industry practice is to add specialty chemicals directly to the caustic tower to prevent or minimize formation of polymer-based fouling materials or aid in their removal from the tower.
However, caustic wash solutions can be entrained into the compressor section of an ethylene plant. Often, the caustic tower is located between two stages of the plant's compression section. For example, if the plant's compression section consists of four stages, the caustic tower may be located between the third and fourth stages. If a problem occurs in the caustic tower, foaming for example, caustic wash solution may be carried into the succeeding stage (the fourth stage in the example) of compression. It is known that approximately 20-30% of acetaldehyde which enters the caustic tower is distilled overhead and proceeds to the next stage of compression. Entrained caustic wash solution reacts with acetaldehyde which is carried into this succeeding stage of compression.